This invention relates generally to pistons for use in reciprocating internal combustion engines and more particularly to the piston pin bores of such pistons.
Conventional pistons for use in internal combustion engines, such as high speed diesel engines, generally comprise a crown or head portion which acts as a reaction surface for the combustion in the corresponding cylinder of the engine. Depending from and cast integrally with the head portion of the piston is a skirt for guiding the piston in its reciprocating movement within the cylinder. A pair of bosses also depend from the piston head and are cast integrally with the head and skirt portions. A generally cylindrical piston pin bore extends through each boss, with the two bores being coaxial. Each bore receives an end of a piston pin. A connecting rod is journalled at one of its ends about the piston pin and at its other end to a crankshaft.
In the operation of a typical reciprocating engine, driving forces such as are exerted by expanding gas within the combustion chamber of the engine are transmitted from the piston head through the piston pin bosses to the piston pin. The forces are then transmitted to the connecting rod and then to the driveshaft.
Problems such as piston pin bore cracking can often arise because of the high stress concentration on the piston pin bosses. This cracking can occur because the piston pin has a tendency to flex and flatten as the driving force is carried through it. The pin can thereby become axially disaligned with respect to the piston pin bores, thereby causing a great portion of the driving force to be localized at the inner (i.e., adjacent the connecting rod) and upper (i.e., adjacent the piston head) portions of the bores. The recurrent concentration of forces can cause an overstressing of the boss material surrounding the piston pin bore which may lead to a fatigue failure in the form of cracking.
The aforementioned problems can occur more often when the piston pin bore is not smoothed and prestressed. Roll burnishing has previously been used as a finishing step to smooth the surface and to eliminate some surface defects in the piston pin bore. In heretofore known processes for smoothing by roll burnishing the diameter of the bore is enlarged by a maximum amount of 0.0005 inches and essentially no prestressing or densification of material occurs. When roll burnishing for purposes of smoothing, the generally cylindrical shape of the bore is maintained and no further rounding or reboring is required. However, because this process does not significantly densify or prestress the material around the bore, the probability of cracking under some operating conditions can remain relatively high.
Roll burnishing has also been used to densify or prestress the material around the piston pin bores. Heretofore when a roll burnishing process has been used for densification, the depth of densification of material has been a minimum of about 0.020 inches. In this process, the diameter of the bore is enlarged about 0.004 inches or more.
A problem encountered with the roll burnishing densification process is distortion of the piston pin bore. The end portions of the bore have a tendency to increase in diameter more readily than the center portions. This problem becomes more prominent if the piston is of a material having a high porosity, such as cast aluminum. Distortion occurs during the roll burnishing process when the material surrounding the bore is squeezed or compressed, causing material at the edges of the bore to flow axially. The distortion occuring as a result of the heretofore employed roll burnishing process for densification has left the bore in a relatively exaggerated and presently unacceptable hourglass shape. A reaming or reboring process is then required as a finishing step.
The reaming or reboring process again places the bore into its original cylindrical shape. Because the roll burnishing process densifies the material to a relatively great depth (0.020 inches or greater), much of the densified material is removed in the reaming process while an acceptable thickness of over about 0.010 inches of prestressed material still remains. However, the reboring is an additional step in the piston manufacturing process that was heretofore necessary when roll burnishing for densification was utilized.